Diabetes mellitus is an ever-increasing threat to human health. For example, in the United States current estimates maintain that about 16 million people suffer from diabetes mellitus. Type II diabetes also known as non-insulin-dependent diabetes mellitus accounts for approximately 90-95% of diabetes cases, killing about 193,000 U.S. residents each year. Type II diabetes is the seventh leading cause of all deaths. In Western societies, type II diabetes currently affects 6% of the adult population with world-wide frequency expected to grow by 6% per annum. Although there are certain inheritable traits that may predispose particular individuals to developing type II diabetes, the driving force behind the current increase in incidence of the disease is the increased sedentary life-style, diet, and obesity now prevalent in developed countries. About 80% of diabetics with type II diabetes are significantly overweight. Also, an increasing number of young people are developing the disease. Type II diabetes is now internationally recognized as one of the major threats to human health in the 21st century.
Type II diabetes manifests as inability to adequately regulate blood-glucose levels and may be characterized by a defect in insulin secretion or by insulin resistance. Namely, those who suffer from Type II diabetes have too little insulin or cannot use insulin effectively. Insulin resistance refers to the inability of the body tissues to respond properly to endogenous insulin. Insulin resistance develops because of multiple factors, including genetics, obesity, increasing age, and having high blood sugar over long periods of time. Type II diabetes, sometimes called mature on set, can develop at any age, but most commonly becomes apparent during adulthood. However, the incidence of type II diabetes in children is rising. In diabetics glucose levels build up in the blood and urine causing excessive urination, thirst, hunger, and problems with fat and protein metabolism. If left untreated, diabetes mellitus may cause life-threatening complications, including blindness, kidney failure, and heart disease.
Type II diabetes is currently treated at several levels. A first level of therapy is through diet and/or exercise, either alone or in combination with therapeutic agents. Such agents may include insulin or pharmaceuticals that lower blood glucose levels. About 49% of individuals with Type II diabetes require oral medications, about 40% require insulin injections or a combination of insulin injections and oral medications, and 10% use diet and exercise alone.
Current therapies include: insulin secretagogues, such as sulphonylureas, which increase insulin production from pancreatic β-cells; glucose-lowering effectors, such as metformin which reduce glucose production from the liver; activators of the peroxisome proliferator-activated receptor γ (PPARγ), such as the thiazolidinediones, which enhances insulin action; and α-glucosidase inhibitors which interfere with gut glucose production. There are, however, deficiencies associated with currently available treatments. For example sulphonylureas and insulin injections can be associated with hypoglycemic episodes and weight gain. Furthermore, patients often lose responsiveness to sulphonylureas over time. Metformin and α-glucosidase inhibitors often lead to gastrointestinal problems and PPARγ agonists tend to cause increased weight gain and edema.
Bile acids (BA) are amphipathic molecules which are synthesized in the liver from cholesterol and stored in the gall bladder until secretion to the duodenum and intestine to play an important role in the solubilization and absorption of dietary fat and lipid-soluble vitamins. Approx. 99% of BA are absorbed again by passive diffusion and active transport in the terminal ileum and transported back to the liver via the portal vein (enterohepatic circulation). In the liver, BA decrease their own biosynthesis from cholesterol through the activation of the farnesoid X receptor alpha (FXRα) and small heterodimer partner (SHP), leading to the transcriptional repression of cholesterol 7α-hydroxylase, the rate-limiting step of BA biosynthesis from cholesterol.
GPBAR1, in the literature termed TGR5, M-BAR or BG37 as well, was recently identified as a G-protein coupled receptor (GPCR) responsive to BA (Kawamata et al., J. Biol. Chem. 2003, 278, 9435-9440; Maruyama et al., Biochem. Biophys. Res. Commun. 2002, 298, 714-719). GPBAR1 is a G(alpha)s-coupled GPCR and stimulation by ligand binding causes activation of adenylyl cyclase which leads to the elevation of intracellular cAMP and subsequent activation of downstream signaling pathways. The human receptor shares 86, 90, 82, and 83% amino acid identity to bovine, rabbit, rat, and mouse receptor, respectively. GPBAR1 is abundantly expressed in the intestinal tract, monocytes and macrophages, lung, spleen, placenta (Kawamata et al., J. Biol. Chem. 2003, 278, 9435-9440). BA induced receptor internalization, intracellular cAMP production and activation of extracellular signal-regulated kinase in GPBAR1-expressing HEK293 and CHO cells.
GPBAR1 was found to be abundantly expressed in monocytes/macrophages from humans and rabbits (Kawamata et al., J. Biol. Chem. 2003, 278, 9435-9440), and BA treatment suppressed LPS-induced cytokine production in rabbit alveolar macrophages and human THP-1 cells expressing GPBAR1. These data suggest that bile acids can suppress the macrophage function via activation of GPBAR1. In the liver functional GPBAR1 was found in the plasma membranes of Kupffer cells, mediating inhibition of LPS-induced cytokine expression (Keitel, Biochem. Biophys. Res. Commun. 2008, 372, 78-84), and of sinusoidal endothelial cells, where bile salts led to an increase in intracellular cAMP and to the activation and enhanced expression of the endothelial nitric oxide (NO) synthase (Keitel, Hepatology 2007, 45, 695-704). Furthermore, GPBAR1 has been detected in cholangiocytes of rat liver (Keitel, Biochem. Biophys. Res. Commun. 2008, 372, 78-84). Hydrophobic bile acids, such as taurolithocholic acid, increase cAMP in cholangiocytes suggesting that GPBAR1 may modulate ductal secretion and bile flow. In summary, GPBAR1 agonists may trigger a protective as well as medicative mechanism in cholestatic livers.
GPBAR1 is expressed in intestinal enteroendocrine cell lines from human (NCI-H716) and murine (STC-1, GLUTag) origin (Maruyama et al., Biochem. Biophys. Res. Commun. 2002, 298, 714-719). Stimulation of GPBAR1 by BA stimulated cAMP production in NCI-H716 cells. Intracellular increases in cAMP suggested that BA may induce the secretion of glucagon-like peptide-1 (GLP-1). Indeed, activation of GPBAR1 by BA promoted GLP-1 secretion in STC-1 cells (Katsuma et al., Biochem. Biophys. Res. Commun. 2005, 329, 386-390). Receptor-specificity has been demonstrated by RNA interference experiments which revealed that reduced expression of GPBAR1 resulted in diminished secretion of GLP-1. There is compelling evidence that GPBAR1-mediated GLP-1 release extends to in vivo. In the isolated vascularly perfused rat colon, BAs have been shown to trigger GLP-1 secretion (Plaisancie et al., J. Endocrin. 1995, 145, 521-526). In humans, intracolonic administration of deoxycholate showed marked increases in plasma levels of GLP-1 and the co-secreted PYY (Adrian et al., Gut 1993, 34, 1219-1224).
GLP-1 is a peptide secreted from enteroendocrine L cells has been shown to stimulate insulin release in glucose dependent manner in humans (Kreymann et al., Lancet 1987, 2, 1300-1304) and studies in experimental animals demonstrated that this incretin hormone is necessary for normal glucose homeostasis. In addition, GLP-1 can exert several beneficial effects in diabetes and obesity, including 1) increased glucose disposal, 2) suppression in glucose production, 3) reduced gastric emptying, 4) reduction in food intake and 5) weight loss. More recently, much research has been focused on the use of GLP-1 in the treatment of conditions and disorders such as diabetes mellitus, stress, obesity, appetite control and satiety, Alzheimer disease, inflammation, and diseases of the central nervous system. (see, for example, Bojanowska et al., Med. Sci. Monit. 2005, 8, RA271-8; Perry et al., Current Alzheimer Res. 2005, 3, 377-385; and Meier et al., Diabetes Metab. Res. Rev. 2005, 2, 91-117). However, the use of a peptide in clinical treatment is limited due to difficult administration, and in vivo stability. Therefore, a small molecule that either mimics the effects of GLP-1 directly, or increases GLP-1 secretion, may be useful in treatment of the variety of conditions or disorders described above, namely diabetes mellitus.
Furthermore, activation of GPBAR1 might be beneficial for the treatment of obesity and metabolic syndrome. Mice fed a high fat diet (HFD) containing 0.5% cholic acid gained less weight than control mice on HFD alone independent of food intake (Watanabe et al., Nature 2006, 439, 484-489). These effects were independent of FXR-alpha, and are likely to results from the binding of BA to GPBAR1. The proposed GPBAR1-mediated mechanism is leading to the subsequent induction of the cAMP-dependent thyroid hormone activating enzyme type 2 (D2) which converts the inactive T3 into the active T4, resulting in the stimulation of the thyroid hormone receptor and promoting energy expenditure. Mice lacking the D2 gene were resistant to cholic acid-induced weight loss. In both rodents and humans, the most thermogenically important tissues (the brown adipose and skeletal muscle) are specifically targeted by this mechanism because they co-express D2 and GPBAR1. The BA-GPBAR1-cAMP-D2 signalling pathway is therefore a crucial mechanism for fine-tuning energy homeostasis that can be targeted to improve metabolic control.